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United States Patent |
5,610,222
|
Mills
,   et al.
|
March 11, 1997
|
Polymeric film
Abstract
A polymeric film comprising from 0.0005% to 2% by weight, based upon the
weight of the polymer in the film, of filler particles having a volume
distributed median particle diameter of from 0.1 to 12.5 .mu.m, the filler
particles being obtainable by calcining precursor silicone resin particles
prior to incorporation into the film polymer.
Inventors:
|
Mills; Paul D. A. (Darlington, GB);
Siddiqui; Junaid A. (Richmond, VA);
Rakos; Karl (Chilton, GB)
|
Assignee:
|
Imperial Chemical Industries PLC (London, GB2)
|
Appl. No.:
|
312757 |
Filed:
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September 27, 1994 |
Current U.S. Class: |
524/493; 524/539; 524/604; 524/605; 525/446; 525/474 |
Intern'l Class: |
C08K 003/34 |
Field of Search: |
524/493,604,605,539
525/474,446
|
References Cited
U.S. Patent Documents
4761327 | Aug., 1988 | Hamano et al. | 524/539.
|
4849564 | Jul., 1989 | Shimizu et al. | 525/476.
|
4921670 | May., 1990 | Dallmann et al. | 525/474.
|
5137939 | Aug., 1992 | Siddiqui | 524/493.
|
Foreign Patent Documents |
0229670 | Oct., 1992 | EP.
| |
64-11135 | Jan., 1989 | JP | 524/493.
|
838708 | Jun., 1960 | GB.
| |
Other References
Derwent Publications, Ltd., London, GB, Abstract JP-A-63 017 962, Jan. 25,
1988.
|
Primary Examiner: Sweet; Mark D.
Attorney, Agent or Firm: Cushman Darby & Cushman IP Group Pillsbury Madison & Sutro LLP
Parent Case Text
This application is a divisional application of Ser. No. 08/271,138 filed
Jul. 7, 1994.
Claims
We claim:
1. A method of producing a polymeric film comprising the steps of:
(i) calcining, at a temperature of at least 250.degree. C., precursor
silicone resin particles to produce calcined filler particles having a
volume distributed median particle diameter of from 0.1 to 12.5 .mu.m,
(ii) mixing the calcined filler particles with polymer or polymer-forming
material at a concentration of 0.0005% to 2% by weight, based upon the
weight of the polymer, and
(iii) extruding the polymer/calcined filler particle mixture to form a
film.
2. A method according to claim 1 wherein the film is oriented by drawing in
two mutually perpendicular directions.
3. A method according to claim 1 wherein the precursor silicone resin
particles comprise a three-dimensional polymer chain structure of the
formula
R.sub.x SiO.sub.2-(x/2)
wherein x is greater than or equal to 1, and R represents an organic group.
4. A method according to claim 1 wherein the calcined filler particles
comprise a three-dimensional polymer chain structure of the formula
R.sub.x (OH).sub.y SiO.sub.2-((x+y)/2)
wherein x is in the range from 0 to 0.9, y is in the range from 0 to 1.2,
and R represents an organic group.
5. A method according to claim 4 wherein x is in the range from 0.05 to
0.9.
6. A method according to claim 4 wherein x is in the range from 0.1 to 0.5.
7. A method according to claim 4 wherein y is in the range from 0.2 to 1.0.
8. A method according to claim 1 wherein the precursor silicone resin
particles are heated at a temperature from 300.degree. C. to 350.degree.
C. for 3 to 8 hours.
9. A method according to claim 1 wherein the polymer/calcined filler
particle mixture additionally comprises at least one other additive
conventionally employed in the manufacture of polymeric films.
10. A method according to claim 1 wherein the calcined filler particles
have a BET specific surface area of less than 80 m.sup.2 /g.
11. A method according to claim 1 wherein the calcined filler particles
have a volume distributed median particle diameter in the range from 2.8
to 4.5 .mu.m.
12. A method according to claim 1 wherein the inorganic content of the
calcined filler particles is at least 99.0% of the silica and/or
hydroxylated silica.
13. A method according to claim 1 wherein the polymer is polyethylene
terephthalate and/or polyethylene naphthalate.
Description
This invention relates to a polymeric film, and in particular to a
polymeric film containing filler particles.
It is known that polymeric films often have poor handling properties which
may result in difficulties in winding the films into high quality reels
and inefficient passage through processing, for example, slitting,
equipment. Film handling properties can be improved by increasing the
surface roughness of the film, suitably by the use of coatings, or
alternatively by incorporating fillers, i.e. organic and/or inorganic
particles into the film. A combination of coatings and fillers may be used
to improve film handling properties. The problem with using coatings to
improve film handleability is that they limit the range of uses to which
the film my be applied because of the difficulty in applying additional
coating layers which may be required, for example, to provide antistatic,
adhesion promoting or release properties. Filler incorporated into a
coating layer is susceptible to abrasion and loss from the coating layer.
A wide range of fillers have been incorporated into films to improve
handling properties, such as titanium dioxide, calcium carbonate, glass,
barium sulphate, silica, kaolin, china clay, zeeospheres and calcium
phosphates.
There is a requirement for filler particles to be of uniform particle size,
and preferably spherical in shape in order to produce a film having a
uniform surface roughness. Various types of silica particles are
commercially available. However, the aforementioned silica particles are
generally lacking in at least one property required, such as particle size
and uniformity thereof, spherical shape, of a filler to meet the strict
requirements of many polymeric film applications. In particular, there is
a lack of commercial availability of uniform spherical particles of silica
of relatively large particle size, for example greater than 2 to 3 .mu.m.
European Patent No 229670 discloses a polyester film comprising 0.005% to
1% of silicone resin particles having an average particle diameter of 0.01
.mu.m to 4 .mu.m. The film is used in magnetic recording media.
Unfortunately, the silicone resin particles disclosed therein are very
hydrophobic and can be difficult to incorporate uniformly into polymeric
films.
The presence of fillers in polymeric films result in a depreciation in the
optical clarity and an increase in the haze, of the film.
Optical clarity and transparency are important criteria in a wide range of
film applications, such as packaging, metallised films, reprographic films
and films for general industrial use. There is a continuing need for films
exhibiting high light transmittance, low haze and excellent handling
properties.
We have surprisingly reduced or substantially overcome one or more of the
aforementioned problems.
Accordingly, the present invention provides a polymeric film comprising
from 0.0005% to 2% by weight, based upon the weight of the polymer in the
film, of filler particles having a voltune distributed median particle
diameter of from 0.1 to 12.5 .mu.m, the filler particles being obtainable
by calcining precursor silicone resin particles prior to incorporation
into the film polymer.
The invention also provides a method of producing a polymeric film
comprising from 0.0005% to 2% by weight, based upon the weight of the
polymer in the film, of filler particles having a volume distributed
median particle diameter of from 0.1 to 12.5 .mu.m, wherein the filler
particles are prepared by calcining precursor silicone resin particles,
prior to incorporation into the film polymer, at a temperature of at least
250.degree. C.
The polymeric film is a self-supporting film, i.e. a self-supporting
structure capable of independent existence in the absence of a supporting
base.
The polymeric film according to the invention may be formed from any
synthetic, film-forming, polymeric material. Suitable thermoplastics,
synthetic, materials include a homopolymer or a copolymer of a 1-olefine,
such as ethylene, propylene or butene-1, a polyamide, a polycarbonate, and
particularly a synthetic linear polyester which my be obtained by
condensing one or more dicarboxylic acids or their lower alkyl (up to 6
carbon atoms) diesters, e.g. terephthalic acid, isophthalic acid, phthalic
acid, 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, succinic acid,
sebacic acid, adipic acid, azelaic acid, 4,4'-diphenyldicarboxylic acid,
hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane (optionally
with a monocarboxylic acid, such as pivalic acid) with one or more
glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl
glycol and 1,4-cyclohexanedimethanol. A polyethylene terephthalate or
polyethylene naphthalate film is preferred. A polyethylene terephthalate
film is particularly preferred, especially such a film which has been
biaxially oriented by sequential stretching in two mutually perpendicular
directions, typically at a temperature in the range 70.degree. to
125.degree. C., and preferably heat set, typically at a temperature in the
range 150.degree. to 250.degree. C., for example as described in British
patent 838708.
The polymeric film may also comprise a polyarylether or thio analogue
thereof, particularly a polyaryletherketone, polyarylethersulphone,
polyaryletheretherketone, polyaryletherethersulphone, or a copolymer or
thioanalogue thereof. Examples of these polymers are disclosed in
EP-A-1879, EP-A-184458 and U.S. Pat. No. 4,008,203. The polymeric film may
comprise a poly(arylene sulphide), particularly poly-p-phenylene sulphide
or copolymers thereof. Blends of the aforementioned polymers may also be
employed.
Suitable thermoset resin polymeric materials include
addition-polymerisation resins--such as acrylics, vinyls, bis-maleimides
and unsaturated polyesters; formaldehyde condensate resins--such as
condensates with urea, melamine or phenols, cyanate resins, functionalised
polyesters, polyamides or polyimides.
The polymeric film according to the invention may be unoriented, or
uniaxially oriented, but is preferably biaxially oriented by drawing in
two mutually perpendicular directions in the plane of the film to achieve
a satisfactory combination of mechanical and physical properties.
Simultaneous biaxial orientation may be effected by extruding a polymeric
tube which is subsequently quenched, reheated and then expanded by
internal gas pressure to induce transverse orientation, and withdrawn at a
rate which will induce longitudinal orientation. Sequential stretching may
be effected in a stenter process by extruding the thermoplastics polymer
as a flat extrudate which is subsequently stretched first in one direction
and then in the other mutually perpendicular direction. Generally, it is
preferred to stretch firstly in the longitudinal direction, i.e. the
forward direction through the film stretching machine, and then in the
transverse direction. A stretched film my be, and preferably is,
dimensionally stabilised by heat-setting under dimensional restraint at a
temperature above the glass transition temperature thereof.
Filler particles, for incorporation into a polymeric film according to the
invention, can be suitably prepared by calcining precursor silicone resin
particles. The precursor silicone resin particles preferably comprise a
three-dimensional polymer chain structure of the formula
R.sub.x SiO.sub.2-(x/2)
wherein x is greater than or equal to 1, preferably from 1 to 1.9, more
preferably 1 to 1.5, and particularly 1 to 1.2. R represents an organic
group, such as an aliphatic hydrocarbon, e.g. methyl, ethyl or butyl
group, or an aromatic hydrocarbon, e.g. phenyl group, or an unsaturated
hydrocarbon, e.g. vinyl group. In a preferred embodiment of the invention
R represents a hydrocarbon group having from 1 to 8, more preferably 1 to
5 carbon atoms. In a particularly preferred embodiment of the invention R
represents a methyl group. R may comprise a mixture of any two or more of
the aforementioned hydrocarbon groups. Particularly preferred precursor
silicone resin particles comprise methyl sesquioxane.
The precursor silicone resin particles suitably have a cross-linked network
of siloxane linkages, comprising a mixture of the structures
##STR1##
wherein R is as defined above.
Suitable precursor silicone resin particles are commercially available, for
example "Tospearl" silicone resin particles available from Toshiba
Silicone Co Ltd, Japan.
Calcining of precursor silicone resin particles results in elimination of
the organic K group and hence a reduction in the value of x in the formula
R.sub.x SiO.sub.2-(x/2). When all the organic material has been removed
x=0, and the result is silica particles (SiO.sub.2).
Calcining is suitably achieved by heating the precursor silicone resin
particles at a temperature greater than 250.degree. C., preferably from
270.degree. C. to 650.degree. C., more preferably from 280.degree. C. to
500.degree. C., particularly from 290.degree. C. to 400.degree. C., and
especially 300.degree. C. to 350.degree. C. The silicone resin particles
are preferably heated for at least 1 hour, more preferably for 2 to 12
hours, particularly 3 to 8 hours, and especially 3 to 5 hours. The
silicone resin particles are preferably heated in an oven in an atmosphere
of air, or alternatively in a suitable inert atmosphere, such as nitrogen
gas.
Elimination of the organic material during calcination of the precursor
silicone resin particles results in a reduction in weight of the
particles. It is preferred that the silicone resin particles lose from 0%
to 20%, more preferably up to 10%, particularly up to 5%, and especially
up to 2% of the original weight thereof during the calcination process.
The calcined filler particles for use in a polymeric film according to the
invention optionally contain an organic group. The ratio of organic
groups, preferably methyl, to silicon atoms present in the calcined filler
particles is preferably in the range from 0 to 0.9:1, more preferably 0.05
to 0.7:1, particularly 0.1 to 0.5:1, and especially 0.15 to 0.3:1.
The filler particles suitably comprise a three-dimensional polymer chain
structure of the formula
K.sub.x (OH).sub.y SiO.sub.2-((x+y)/2)
wherein R represents an organic group as defined above for the precursor
silicone resin particles. During calcination, at least in air, elimination
of the R group and formation of Si-OH bonds and additional Si-O-Si bonds
occurs. There are substantially no Si-OH bonds present in the precursor
silicone resin particles. Further calcination results in conversion of the
Si-OH bonds to Si-O-Si bonds and the eventual formation of silica
particles. The value of x is preferably in the range from 0 to 0.9, more
preferably 0.05 to 0.7, particularly 0.1 to 0.5, and especially 0.15 to
0.3. The value of y is preferably in the range from 0 to 1.2, more
preferably from 0.2 to 1.0, particularly 0.4 to 0.8, and especially 0.5 to
0.7. The values of x and y can be determined, for example, by .sup.29 Si
magic angle spinning NMR spectroscopy.
The chemical composition of filler particles for use in the present
invention is preferably from 80% to 100%, more preferably 90% to 99.9%,
especially 92% to 98%, and particularly 94% to 97% by weight of inorganic
material, and correspondingly preferably from 0% to 20%, more preferably
0.1% to 10%, especially 2% to 8%, and particularly 3% to 6% by weight of
organic material. In a preferred embodiment of the invention the organic
component of the filler particles comprises predominantly, and more
preferably substantially, methyl groups. The inorganic component of the
filler particles preferably comprises at least 98%, more preferably at
least 99%, particularly at least 99.5%, and especially at least 99.9% of
silica and/or hydroxylated silica, i.e. of silicon, oxygen and hydrogen
atoms.
In order to obtain the advantageous properties of the present invention the
concentration of filler particles, as defined herein, present in the
polymeric film should be in the range from 0.0005% to 2%, preferably
0.001% to 0.5%, more preferably 0.0025% to 0.1%, particularly 0.004% to
0.02%, and especially 0.005% to 0.01% by weight, based upon the weight of
the polymer in the film. The aforementioned preferred concentration ranges
are particularly applicable to a filled monofilm. However, a polymeric
film according to one embodiment of the invention is a composite film
having a first layer, preferably essentially unfilled, and on at least one
surface thereof a second layer preferably comprising in the range from
0.02% to 0.5%, more preferably 0.04% to 0.3%, and particularly 0.05% to
0.08% by weight of filler particles, based upon the weight of the polymer
in the second layer.
The volume distributed median particle diameter (equivalent spherical
diameter corresponding to 50% of the volume of all the particles, read on
the cumulative distribution curve relating volume % to the diameter of the
particles--often referred to as the "D(v,0.5)" value) of the filler
particles incorporated into the polymeric film according to the invention
is in a range from 0.1 to 12.5 .mu.m, preferably 0.4 to 8.0 .mu.m, more
preferably 0.7 to 6.0 .mu.m, particularly 1.8 to 5.0 .mu.m, and especially
2.8 to 4.5 .mu.m.
The size distribution of the filler particles is also an important
parameter in obtaining a polymeric film having a uniform surface
roughness. The filler particles have a particle size distribution ratio
D.sub.25 /D.sub.75 (where D.sub.25 and D.sub.75, respectively, are the
particle diameter of 25% and 75% of a volume based cumulative particle
size distribution curve) value of from 1.1 to 1.6, preferably 1.15 to 1.5,
more preferably 1.2 to 1.4, and especially 1.25 to 1.35. In a preferred
embodiment of the invention the filler particles also have a particle size
distribution ratio D.sub.10 /D.sub.90 (where D.sub.10 and D.sub.90,
respectively, are the particle diameter of 10% and 90% of a volume based
cumulative particle size distribution curve) value of from 1.2 to 2.2,
preferably 1.3 to 2.0, more preferably 1.5 to 1.9, and especially 1.7to
1.8.
The presence of excessively large filler particles can result in the film
exhibiting unsightly `speckle`, i.e. where the presence of individual
filler particles in the film can be discerned with the naked eye.
Desirably, therefore, the actual particle size of 99.9% by volume of the
particles should not exceed 20 .mu.m, and preferably not exceed 15 .mu.m.
Preferably at least 90%, more preferably at least 95% by volume of the
particles are within the range of the volume distributed median particle
diameter .+-.1.5 .mu.m, especially .+-.1.0 .mu.m and particularly
.+-.0.5.mu.m.
Particle sizes of the particles may be measured by electron microscope,
Coulter counter, sedimentation analysis and light scattering, preferably
techniques based on laser light diffraction.
The filler particles for use in the present invention are of substantially,
circular cross-section irrespective of the selected viewing point. The
particles exhibit an average aspect ratio d.sub.1 :d.sub.2 (where d.sub.1
and d.sub.2, respectively, are the maximum and minimum dimensions of the
particle) of from 1:1 to 1:0.9, preferably from 1:1 to 1:0.95, and
especially from 1:1 to 1:0.98. The aspect ratio of a filler particle can
be determined by measuring the d.sub.1 and d.sub.2 value of a filler
particle selected from a photographic image obtained by using a scanning
electron microscope. An average aspect ratio can be obtained by
calculating the mean value of 100 typical filler particles.
In a particularly preferred embodiment of the invention, the filler
particles have a BET specific surface area, measured as described herein,
of less than 80, more preferably in the range from 3 to 50, particularly 5
to 45, and especially 15 to 40 m.sup.2 /g.
The filler particles preferably have a skeletal density, measured as
described herein, in the range from 1.95 to 2.3, more preferably 2.00 to
2.2, and particularly 2.05 to 2.15 g/cm.sup.3.
The filler particles may be added to the polymeric layer or polymeric
layer-forming material at any point in the film manufacturing process
prior to the extrusion of the polymer. For example, in the production of a
preferred polyester film, the particles may be added during monomer
transfer or in the autoclave, although it is preferred to incorporate the
particles as a glycol dispersion during the esterification reaction stage
of the polyester synthesis. Alternatively, the particles may be added as a
dry powder into the polymer melt via a twin-screw extruder or by
masterbatch technology.
The polymeric film of the present invention is suitably transparent,
preferably having a wide angle haze, measured as described herein, for a
75 .mu.m thick film, of <10%, more preferably <5%, especially <2%, and
particularly <1%.
The surface of a polymeric film according to the invention preferably
exhibits a static coefficient of friction, measured as descibed herein,
when measured against itself, of <0.9, preferably <0.7, especially <0.5,
and particularly <0.4.
The layers of a film according to the invention may conveniently contain
any of the additives conventionally employed in the manufacture of
polymeric films. Thus, agents such as dyes, pigments, lubricants,
anti-oxidants, anti-blocking agents, surface active agents, slip aids,
gloss-improvers, prodegradants, ultra-violet light stabilisers, viscosity
modifiers and dispersion stabilisers may be incorporated into the
polymeric film layer(s), as appropriate. The additives will preferably not
increase the wide angle haze of the polymeric film up to or above the
aforementioned values.
A polymeric film according to the invention may be coated on one or both
surfaces with one or more additional coating, ink, lacquer and/or metal
layers, for example to form a laminate or composite which exhibits
improved properties, such as antistatic, adhesion promoting or release,
compared with the component materials. A preferred antistatic coating
layer comprises a quaternary ammonium compound, preferably in combination
with an acrylic resin.
Prior to the deposition of a coating medium onto the polymeric film, the
exposed surface thereof may, if desired, be subjected to a chemical or
physical surface-modifying treatment to improve the bond between that
surface and the subsequently applied coating layer. A preferred treatment
is corona discharge, which may be effected in air at atmospheric pressure
with conventional equipment using a high frequency, high voltage
generator, preferably having a power output of from 1 to 20 kw at a
potential of 1 to 100 kv. Discharge is conveniently accomplished by
passing the film over a dielectric support roller at the discharge station
at a linear speed preferably of 1.0 to 500 m per minute. The discharge
electrodes may be positioned 0.1 to 10.0 mm from the moving film surface.
Alternatively, the surface of the film my be pretreated with an agent
known in the art to have a solvent or swelling action on the polymeric
layer. Examples of such agents which are particularly suitable for the
treatment of a polyester film surface include a halogenated phenol
dissolved in a common organic solvent e.g. a solution of
p-chloro-m-cresol, 2,4-dichlorophenol, 2,4,5- or 2,4,6-trichlorophenol or
4-chlororesorcinol in acetone or methanol.
The coating medium may be applied to an already oriented polymeric film
surface, but application of the coating medium is preferably effected
before or during the stretching operation.
In particular, it is preferred that the coating medium should be applied to
the film surface between the two stages (longitudinal and transverse) of a
thermoplastics film biaxial stretching operation. Such a sequence of
stretching and coating is especially preferred for the production of a
coated polyester film comprising polyethylene terephthalate, which is
preferably firstly stretched in the longitudinal direction over a series
of rotating rollers, coated with the coating layer, and then stretched
transversely in a stenter oven, preferably followed by heat setting.
Polymeric films according to the invention are suitable for use in a wide
range of film applications, such as packaging, e.g. as carton windows,
metallised films, reprographic films and films for general industrial use.
Polymeric films described herein are particularly suitable for information
storage and display, such as imaging, montage, masking, stencil, overhead
projection, membrane touch switch, microfilm and printing, such as thermal
wax transfer printing.
In this specification the following test methods have been used to
determine certain properties of the filler particles and polymeric film:
Filler Particle Analysis
Volume distributed median particle diameter, and particle size distribution
ratios D.sub.25 /D.sub.75 and D.sub.10 /D.sub.90 were measured using a
Coulter LS130 (Coulter Electronics Ltd, Luton, UK) particle sizer.
BET specific surface area was measured by multi-point nitrogen adsorption
using a Micromeritics ASAP 2400 (Micromeritics Limited, Dunstable, UK).
Relative pressures between 0.05 and 0.21 were used, and the outgassing
conditions were 1 hour at 140.degree. C. with nitrogen purge (1 to 2
liters/hour).
Skeletal density was measured by helium pycnometry using a Micromeritics
Accupyc 1330 (Micromeritics Limited, Dunstable, UK).
The ratio of methyl groups to silicon atoms was measured by .sup.29 Si
magic angle spinning NMR spectroscopy. The spectrum was acquired on a
Bruker MSL200 NMR spectrometer operating at a frequency of 39.73 MHz for
.sup.29 Si. The magic angle was set using KBr and the spinning speed was
5050 Hz. The NMR free induction decay consisting of 2K complex data points
was acquired using the single pulse excitation pulse sequence together
with high power 1H decoupling where the 1H decoupling field was of the
order of 70 kHz. The spectral width was 20 kHz, .sup.29 Si pulse length
5.5 .mu.s (90.degree.) and recycle delay 60 s. 1000 transients were
accumulated. Data processing consisted of apodisation using an exponential
with the Bruker LB parameter equal to 60 Hz, followed by Fourier
transformation, phasing, baseline correction and integration using the
Bruker software EP-I routine.
Polymeric Film Analysis
The static coefficient of friction of the polymeric film was measured
against itself by an inclined plane method based on ASTM test D 4518-87,
using a Model IPST (Specialist Engineering, Welwyn, UK).
Wide angle haze was determined as the percentage of transmitted light which
deviates from the normal to the surface of the film by an average amount
greater than 2.5.degree. of arc during passage through the film,
essentially according to ASTM test D 1003-61, using a Hazegard XL211
Hazemeter (BYK Gardner, US).
The handling and winding properties of the film were evaluated on a
slitting machine. Reels of length between 1000 m and 3000 m, and width
between 500 mm and 2000 mm were slit at speeds between 50 and 400 meters
per minute. The resultant slit reels were assessed for their physical
appearance.
The invention is illustrated by reference to the following
EXAMPLE 1
Precursor silicone resin particles (Tospearl 145, supplied by Toshiba
Silicone Co Ltd, Japan) were calcined by heating in an oven at 300.degree.
C. for 4 hours in an atmosphere of air in order to produce filler
particles for use in the present invention. The resultant filler particles
exhibited the following characteristics which were measured using the
methods described herein:
(i) volume distributed median particle diameter=4.4 .mu.m.
(ii) particle size distribution ratio D.sub.25 /D.sub.75 =1.40
(iii) particle size distribution ratio D.sub.10 /D.sub.90 =1.85
(iv) BET specific surface area=45 m.sup.2 /g
(v) skeletal density=2.06 g/cm.sup.3
(vi) ratio of methyl groups to silicon atoms=0.2
Polyethylene terephthalate polymer containing approximately 600 ppm of
filler particles, produced as described above by calcining precursor
silicone resin particles, was extruded through a film-forming die onto a
water cooled rotating, quenching drum to yield an amorphous cast composite
extrudate. The cast extrudate was heated to a temperature of about
80.degree. C. and then stretched longitudinally at a forward draw ratio of
3.2:1. The polymeric film was passed into a stenter oven, where the film
was stretched in the sideways direction to approximately 3.4 times its
original dimensions. The biaxially stretched polymeric film was heat set
at a temperature of about 225.degree. C. Final film thickness was
approximately 188 .mu.m.
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